This invention relates to filters and specifically to fibrous depth filters of the kind in which one or more preformed, integral filter elements are fitted into a housing. Such filters are inter alia encountered in the filtration of blood products, and are described, for example, in copending U.S. patent application Ser. No. 07/259,773 filed on Oct. 19, 1988 and now U.S. Pat. No. 4,925,572.
Depth filters are capable of removing specified components of a mixture with a high level of efficiency. It will be apparent, however, that this efficiency will be of little value if the component to be filtered is able to escape by passing around the edges of, rather than through, the tangle of fibers that forms the depth filter.
It is known to seek to avoid this result by forming the filter element actually in the barrel of the device by forcing a mass of fibers into an elongated cylindrical container. Because it is difficult to manipulate fibers in a uniform reproducible manner, this can, in addition to resulting in a filter with uneven filtering characteristics, allow bypassing adjacent the wall of the container.
Filters have also been made commercially in which multiple layers of media in sheet form have been edge sealed by compression between internal flanges. Such assemblies tend to be poorly sealed unless the flange is of sufficient width, in which case a substantial proportion of the filter area is lost because fluid access to the peripheral areas under the flange is poor.
Copending U.S. application Ser. No. 07/259,773 (referred to above and called hereinafter '773) is directed to a leucocyte depletion filter with an efficiency of 99.9 to 99.99% and uses a filter disc in the form of a right circular cylinder assembled into a cylindrical housing of lateral dimension 0.1 to 1% smaller than the filter disc, i.e., into a housing with an interference fit.
The problems can be best appreciated by considering the requirement of obtaining efficiencies in the range of 99.999 to 99.99999% in removing leucocytes from red cell concentrate during blood processing. Filters for this purpose tend to be comparatively thick, for example, in excess of 3 mm, and may comprise in addition a prefilter of thickness of up to 1 mm or more. In order to be effective, a flange 4 mm wide may be needed; in a typical filter with 60 cm.sup.2 of effective flow area, the effective area lost under the flange would be about 9 cm.sup.2, or about 15% of the total area.
This invention is directed to an edge sealing system compatible with filters with efficiencies up to about 99.9999%, for which an interference fit such as is described in the '773 application provides sealing that tends to be inadequate.
This invention provides an edge sealing method that permits the consistent production of filters of a constant uniform quality and performance, and with excellent resistance to bypassing around the elements.